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LTC3857EUHPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3857EUHPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 38 page  LTC3857 3857fa operaTion (Refer to the Functional Diagram) Main Control Loop TheLTC3857usesaconstantfrequency,currentmode step-downarchitecturewiththetwocontrollerchannels operating 180 degrees out of phase. During normal operation,eachexternaltopMOSFETisturnedonwhen theclockforthatchannelsetstheRSlatch,andisturned offwhenthemaincurrentcomparator,ICMP,resetsthe RSlatch.ThepeakinductorcurrentatwhichICMPtrips andresetsthelatchiscontrolledbythevoltageontheITH pin,whichistheoutputoftheerroramplifier,EA.Theerror amplifiercomparestheoutputvoltagefeedbacksignalat theVFBpin,(whichisgeneratedwithanexternalresistor divider connected across the output voltage, VOUT, to ground)totheinternal0.800Vreferencevoltage.Whenthe loadcurrentincreases,itcausesaslightdecreaseinVFB relativetothereference,whichcausestheEAtoincrease theITHvoltageuntiltheaverageinductorcurrentmatches thenewloadcurrent. AfterthetopMOSFETisturnedoffeachcycle,thebottom MOSFETisturnedonuntileithertheinductorcurrentstarts toreverse,asindicatedbythecurrentcomparatorIR,or thebeginningofthenextclockcycle. INTVCC/EXTVCC Power PowerforthetopandbottomMOSFETdriversandmost otherinternalcircuitryisderivedfromtheINTVCCpin.When theEXTVCCpinisleftopenortiedtoavoltagelessthan 4.7V,theVINLDO(lowdropoutlinearregulator)supplies 5.1VfromVINtoINTVCC.IfEXTVCCistakenabove4.7V, theVINLDOisturnedoffandanEXTVCCLDOisturnedon. Onceenabled,theEXTVCCLDOsupplies5.1VfromEXTVCC toINTVCC.UsingtheEXTVCCpinallowstheINTVCCpower tobederivedfromahighefficiencyexternalsourcesuch asoneoftheLTC3857switchingregulatoroutputs. Each top MOSFET driver is biased from the floating bootstrapcapacitor,CB,whichnormallyrechargesduring eachcyclethroughanexternaldiodewhenthetopMOSFET turnsoff.Iftheinputvoltage,VIN,decreasestoavoltage closetoVOUT,theloopmayenterdropoutandattempt toturnonthetopMOSFETcontinuously.Thedropout detectordetectsthisandforcesthetopMOSFETofffor aboutonetwelfthoftheclockperiodeverytenthcycleto allowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and TRACK/ SS1, TRACK/SS2 Pins) ThetwochannelsoftheLTC3857canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3857drawsonly8µA ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8V.TheRUNpinhasaninternal11Vvoltageclampthat allowstheRUNpintobeconnectedthrougharesistortoa highervoltage(forexample,VIN),solongasthemaximum currentintotheRUNpindoesnotexceed100µA. Thestart-upofeachcontroller’soutputvoltageVOUTis controlledbythevoltageontheTRACK/SSpinforthat channel.WhenthevoltageontheTRACK/SSpinisless thanthe0.8Vinternalreference,theLTC3857regulates theVFBvoltagetotheTRACK/SSpinvoltageinsteadofthe 0.8Vreference.ThisallowstheTRACK/SSpintobeused toprogramasoft-startbyconnectinganexternalcapacitor fromtheTRACK/SSpintoSGND.Aninternal1µApull-up currentchargesthiscapacitorcreatingavoltagerampon theTRACK/SSpin.AstheTRACK/SSvoltageriseslinearly from0Vto0.8V(andbeyonduptotheabsolutemaximum ratingof6V),theoutputvoltageVOUTrisessmoothlyfrom zerotoitsfinalvalue. AlternativelytheTRACK/SSpincanbeusedtocausethe start-upofVOUTtotrackthatofanothersupply.Typically, thisrequiresconnectingtotheTRACK/SSpinanexternal resistor divider from the other supply to ground (see ApplicationsInformationsection). |
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